Method for making an abrasion-resistant steel plate and plate obtained

ABSTRACT

The invention concerns a method for making an abrasion-resistant steel part consisting of 0.1%≦C≦0.23%; 0%≦Si≦2%; 0%≦AI≦2%; 0.5%≦Si+AI≦2%; 0%≦Mn≦2.5%; 0%≦Ni≦5%; 0%≦Cr≦5%; 0%≦Mo≦1%; 0%≦W≦2%; 0.05%≦Mo+W/2≦1%; 0%≦B≦0.02%; 0%≦Ti≦0.67%; 0%≦Zr≦1.34%; 0.05%≦Ti+Zr/2≦0.67%; 0%≦S≦0.15%; N&lt;0.030, optionally 0% to 1.5% of Cu; optionally Nb, Ta and V such that Nb/2+Ta/4+V≦0.5 %; optionally Se, Te, Ca, Bi, Pb contents ≦0.1%; the rest being iron and impurities. Additionally: 0.095%≦C*=C−Ti/4−Zr/8+7×N/8, Ti+Zr/2−7×N/2≦0.05% and 1.05×Mn+0.54×Ni+0.5O×Cr+0.3×(Mo++W1/2) 1/2 +K&gt;1.8, with K=1 if B≧0.0005% and K=0 if B&lt;0.0005%. After austenitization, the method consists in: cooling at a speed &gt;0.5° C./s between AC 3  and T=800−270×C*−90×Mn−37×Ni−70×Cr−83×(Mo+W/2) and about T−50° C.; then cooling at a speed 0.1&lt;Vr&lt;150×ep −1.7  between T and 100° C., (ep=thickness of plate in mm); cooling down to room temperature and optionally planishing. The invention also concerns the resulting plate.

The present invention relates to an abrasion-resistant steel and itsproduction method.

Steels for abrasion are known which have a hardness in the order of 400Brinell and which contain approximately 0.15% of carbon, as well asmanganese, nickel, chromium and molybdenum at contents of less than afew % in order to have sufficient quenchability. These steels arequenched so as to have a completely martensitic structure. They have theadvantage of being relatively simple to use by means of welding, cuttingor bending. However, they have the disadvantage of having limitedabrasion resistance. Of course, it is known to increase the abrasionresistance by increasing the carbon content, and therefore the hardness.However, this method of operation has the disadvantage of impairing thesuitability for use.

The object of the present invention is to overcome these disadvantagesby providing an abrasion-resistant steel plate which, all thingsotherwise being equal, has abrasion resistance which is better than thatof the known steels which have a hardness of 400 Brinell, whilst havinga suitability for use which is comparable to that of those steels.

To this end, the invention relates to a method for producing aworkpiece, and in particular a plate, of steel for abrasion whosechemical composition comprises, by weight:0.1%≦C<0.23%0%≦Si≦2%0%≦Al≦2%0.5%≦Si+Al≦2%0%≦Mn≦2.5%0%≦Ni≦5%0%≦Cr≦5%0%≦Mo≦1%0%≦W≦2%0.05%≦Mo+W/2≦1%0%≦Cu≦1.5%0%≦B≦0.02%0%≦Ti≦0.67%0%≦Zr≦1.34%0.05%<Ti+Zr/2≦0.67%0%≦S≦0.15%N<0.03%

-   -   optionally at least one element selected from Nb, Ta and V at        contents such that Nb/2+Ta/4+V≦0.5%,    -   optionally at least one element selected from Se, Te, Ca, Bi, Pb        at contents which are less than or equal to 0.1%, the balance        being iron and impurities resulting from the production        operation, the chemical composition further complying with the        following relationships:        C*=C−Ti/4−Zr/8+7×N/8≧0.095%        and:        Ti+Zr/2−7×N/2≧0.05%        and:    -   1.05×Mn+0.54×Ni+0.50×Cr+0.3×(Mo+W/2)^(1/2)+K>1.8 or more        advantageously 2    -   with: K=1 if B≧0.0005% and K=0 if B<0.0005%, the steel having a        structure which is constituted by martensite or an admixture of        martensite and auto-tempered bainite, the structure further        containing carbides and from 5% to 20% of austenite.

According to the method, the workpiece or the plate is subjected to athermal quenching processing operation which is carried out in the heatfor forming in the hot state, such as rolling, or after austenitizationby reheating in a furnace, which consists in:

-   -   cooling the plate at a mean cooling rate greater than 0.5° C./s        between a temperature greater than AC₃ and a temperature of        approximately from T=800−270×C*−90×Mn−37×Ni−70×Cr−83×(Mo+W/2) to        T−50° C., the temperature being expressed in ° C. and the        contents of C*, Mn, Ni, Cr, Mo and W being expressed as % by        weight,    -   then cooling the plate at a mean core cooling rate        Vr<11.50×ep^(−1.7) (in ° C./s) and greater than 0.1° C./s        between the temperature T and 100° C., ep being the thickness of        the plate expressed in mm,    -   and cooling the plate as far as ambient temperature, planishing        optionally being carried out.

Quenching may optionally be followed by tempering at a temperature ofless than 350° C. and preferably less than 250° C.

The invention also relates to a plate which is obtained in particularusing this method and whose flatness is characterized by a deflectionless than or equal to 12 mm/m and preferably less than 5 mm/m, the steelhaving a structure which is constituted by from 5% to 20% of retainedaustenite, the remainder of the structure being martensitic ormartensitic/bainitic and containing carbides. The thickness of the platemay be from 2 mm to 150 mm.

The hardness is preferably from 280HB to 450HB.

The invention will now be described in greater detail, but in anon-limiting manner, and illustrated with reference to examples.

In order to produce a plate according to the invention, a steel isproduced whose chemical composition comprises, in % by weight:

-   -   more than 0.1% of carbon in order to have a sufficient level of        hardness and in order to allow the formation of carbides, but        less than 0.23%, and preferably less than 0.22% so that the        suitability for welding and cutting is good.    -   From 0% to 0.67% of titanium and from 0% to 1.34% of zirconium,        these contents having to be such that the total Ti+Zr/2 is        greater than 0.05%, preferably greater than 0.1%, and, more        advantageously still, greater than 0.2% so that the steel        contains coarse titanium or zirconium carbides which increase        the abrasion resistance. However, the total Ti+Zr/2 must remain        less than 0.67% because above that level the steel would not        contain sufficient free carbon for the hardness thereof to be        sufficient. Furthermore, the content of Ti+Zr/2 will preferably        be less than 0.50%, or more advantageously 0.40% or 0.30% if        priority needs to be given to the toughness of the material.    -   From 0% (or trace levels) to 2% of silicon and from 0% (or trace        levels) to 2% of aluminium, the total Si+Al being from 0.5% to        2% and preferably greater than 0.7%, or more advantageously,        greater than 0.8%. These elements which are deoxidants, further        have the effect of promoting the production of a metastable        retained austenite which is heavily charged with carbon whose        transformation into martensite is accompanied by a large        expansion promoting the anchoring of the titanium carbides.    -   From 0% (or trace levels) to 2% or even 2.5% of manganese, from        0% (or trace levels) to 4% or even 5% of nickel and from 0% (or        trace levels) to 4% or even 5% of chromium in order to obtain an        adequate level of quenchability and to adjust the various        mechanical characteristics or characteristics for use. Nickel in        particular has an advantageous effect on the toughness, but that        element is expensive. Chromium also forms fine carbides in        martensite or bainite which promote the abrasion resistance.    -   From 0% (or trace levels) to 1% of molybdenum and from 0% (or        trace levels) to 2% of tungsten, the total Mo+W/2 being from        0.05% to 1%, and preferably remaining less than 0.8%, or more        advantageously, less than 0.5%. These elements increase the        quenchability and form fine hardening carbides in the martensite        or bainite, in particular by precipitation owing to        auto-tempering during cooling. It is not necessary to exceed a        content of 1% of molybdenum in order to obtain the desired        effect in particular with regard to the precipitation of        hardening carbides. Molybdenum may be completely or partially        replaced with twice the weight of tungsten. Nevertheless, this        substitution is not desirable in practice since it does not        provide any advantage over molybdenum and is more expensive.    -   Optionally from 0% to 1.5% of copper. That element can bring        about additional hardening without inhibiting the weldability.        Above a level of 1.5%, it no longer has a significant effect,        leads to hot-rolling difficulties and is unnecessarily        expensive.    -   From 0% to 0.02% of boron. This element can be added optionally        in order to increase the quenchability. In order to achieve this        effect, the content of boron must preferably be greater than        0.0005%, or more advantageously, 0.001% and does not need to        exceed substantially. 0.01%.    -   Up to 0.15% of sulphur. That element is a residual which is        generally limited to 0.005% or less, but the content thereof may        be voluntarily increased in order to improve machinability. It        should be noted that in the presence of sulphur, in order to        prevent difficulties concerning transformation in the hot state,        the content of manganese must be greater than seven times the        content of sulphur.    -   Optionally at least one element selected from niobium, tantalum        and vanadium at contents such that Nb/2+Ta/4+V remains less than        0.5% in order to form relatively coarse carbides which improve        the resistance to abrasion. However, the carbides formed by        those elements are less effective than the carbides formed by        titanium or zirconium and, for that reason, they are optional        and added in a limited quantity.    -   Optionally, one or more elements selected from selenium,        tellurium, calcium, bismuth and lead, at contents of less than        0.1% each. Those elements are intended to improve machinability.        It should be noted that, when steel contains Se and/or Te, the        content of manganese must be such, taking into consideration the        content of sulphur, that manganese selenides or tellurides can        form.    -   The balance being iron and impurities resulting from the        production operation. The impurities include in particular        nitrogen, whose content depends on the production method but        does not exceed 0.03% and generally remains less than 0.025%.        Nitrogen may react with titanium or zirconium to form nitrides        which must not be too coarse in order not to inhibit the        toughness. In order to prevent the formation of coarse nitrides,        titanium and zirconium may be added to liquid steel in a very        progressive manner, for example, by placing in contact with the        oxidized liquid steel an oxidized phase, such as a slag charged        with titanium or zirconium oxides, then deoxidizing the liquid        steel in order to cause the titanium or zirconium to diffuse        slowly from the oxidized phase to the liquid steel.

Furthermore, in order to obtain satisfactory properties, the contents ofcarbon, titanium, zirconium and nitrogen are selected such that:C*=C−Ti/4−Zr/8+7×N/8≧0.095%and preferably C*≧0.12% in order to have an increased level of hardnessand therefore better abrasion resistance. The quantity C* represents thecontent of free carbon after precipitation of the titanium and zirconiumcarbides, taking into consideration the formation of titanium andzirconium nitrides. That free carbon content C* must be greater than0.095% in order to have a martensitic or martensitic/bainitic structurehaving sufficient hardness.

Taking into consideration the possible formation of titanium orzirconium nitrides, in order for the quantity of titanium or zirconiumcarbides to be sufficient, the contents of Ti, Zr and N must be suchthat:Ti+Zr/2−7×N/2≧0.05%

The chemical composition is further selected so that the quenchabilityof the steel is sufficient, taking into account the thickness of theplate which it is desirable to produce. To this end, the chemicalcomposition must comply with the relationship:

-   -   Tremp=1.05×Mn+0.54×Ni+0.50×Cr+0.3×(Mo+W/2)^(1/2)+K>1.8 or more        advantageously 2        with: K=1 if B≧0.0005% and K=0 if B<0.0005%.

Furthermore, and in order to obtain good abrasion resistance, themicrographic structure of the steel is constituted by martensite orbainite or-an admixture of those two structures, and from 5% to 20% ofretained austenite. That structure further comprises coarse titanium orzirconium carbides which are formed at high temperature and optionallyniobium, tantalum or vanadium carbides. Owing to the method ofproduction which will be described below, this structure is tempered,with the result that it also comprises molybdenum or tungsten carbidesand optionally chromium carbides.

The inventors have established that the effectiveness of coarse carbidesfor improving abrasion resistance could be inhibited by the prematureseparation thereof and that that separation could be prevented by thepresence of metastable austenite which is transformed under the effectof the abrasion phenomena. The transformation of the metastableaustenite being brought about by expansion, that transformation in theabraded sub-layer increases the resistance to separation of the carbidesand, in that manner, improves abrasion resistance.

Furthermore, the great hardness of the steel and the presence ofembrittling titanium carbides make it necessary to limit insofar aspossible the planishing operations. From that point of view, theinventors established that, by slowing down the cooling sufficiently inthe range of bainitic/martensitic transformation, the residualdeformations of the products are reduced, which allows planishingoperations to be limited. The inventors have established that, bycooling down the workpiece or the plate at a mean core cooling rateVr<1150×ep^(−1.7), (in this formula, ep is the thickness of the plateexpressed in mm and the cooling rate is expressed in ° C./s), below atemperature T=800−270×C*−90×Mn−37×Ni−70×Cr−83×(Mo+W/2), (expressed in °C.), the residual stresses brought about by the phase changes werereduced. That cooling which is slowed down in the bainitic/martensiticrange further has the advantage of bringing about auto-tempering whichcauses the formation of molybdenum, tungsten or chromium carbides andimproves the wear resistance of the matrix which surrounds the coarsecarbides.

In order to produce a very planar plate which has good abrasionresistance and good suitability for use, the steel is produced and castin the form of a slab or bar. The slab or bar is hot-rolled in order toobtain a plate which is subjected to thermal processing which allowsboth the desired structure and a good surface evenness to be producedwithout further planishing or with limited planishing. The thermalprocessing may be carried out in the rolling heat or carried outsubsequently, optionally after cold-planishing or planishing at a mediumtemperature.

In all cases, in order to carry out the thermal processing operation:

-   -   the steel is heated above the point AC₃ in order to confer on it        a structure which is completely austenitic but in which titanium        or zirconium carbides remain,    -   then it is cooled at a mean core cooling rate which is greater        than the critical bainitic transformation velocity as far as a        temperature of from approximately        T=800−270×C*−90×Mn−37×Ni−70×Cr−83×(Mo+W/2) to T−50° C., in order        to prevent the formation of ferritic-perlitic constituents; to        this end, it is generally sufficient to cool at a rate greater        than 0.5° C./s,    -   then, the plate is cooled, between the temperature which has        been defined in this manner (that is to say, approximately from        T to T−50° C.) and approximately 100° C., at a mean core cooling        rate Vr which is less than 1150×ep^(−1.7) and greater than 0.1°        C./s in order to obtain the desired structure,    -   and the plate is cooled as far as ambient temperature,        preferably, but without being compulsory, at a slow rate.

Furthermore, it is possible to carry out a stress-relief processingoperation, such as a tempering operation, at a temperature less than orequal to 350° C., and preferably less than 250° C.

Mean cooling rate is understood to be the cooling rate which is equal tothe difference between the initial and final cooling temperaturesdivided by the cooling time between these two temperatures.

In this manner, a plate is obtained whose thickness can be from 2 mm to150 mm and which has excellent surface evenness, characterized by adeflection of less than 3 mm per metre without planishing or withmoderate planishing. The plate has a hardness of from 280HB to 450HB.That hardness depends principally on the content of free carbonC*=C−Ti/4−Zr/8+7×N/8. The hardness becomes greater as free carboncontent becomes greater. The usability increases as the free carboncontent decreases. With an equal content of free carbon, the resistanceto abrasion becomes higher as the titanium content increases.

By way of example, steel plates 30 mm thick designated A, B, C and Daccording to the invention, E and F according to the prior art, and Gand H given by way of comparison are considered. The chemicalcompositions of the steels, expressed in 10⁻³% by weight, as well as thehardness and a wear resistance index Rus, are summarized in Table 1.TABLE 1 C Si Al Mn Ni Cr Mo W Ti B N HB Rus A 180 550 30 1750 200 1700150 — 150 2 6 360 1.51 B 140 210 610 1450 650 1720 230 120 160 3 7 3451.42 C 220 830 25 1250 220 1350 275 350 2 5 360 2.03 D 158 780 35 1250250 1340 260 110 3 5 363 1.3 E 175 360 25 1720 200 1200 250 — 20 3 5 4201.08 F 150 320 30 1730 250 1260 310 — — 2 6 380 1 G 210 340 25 1230 2601350 280 350 2 5 360 1.11 H 150 320 25 1255 250 1360 260 105 3 6 3660.81

The wear resistance of the steels is measured by the loss of weight of aprismatic test piece which is rotated in a container containing gradedquartzite aggregate for a period of 5 hours.

The wear resistance index Rus of a steel is the ratio of the wearresistance of the steel F, taken by way of reference, and the wearresistance of the steel in question.

The plates A to H are austenitized at 900° C.

After Austenitization:

-   -   the plate of steel A is cooled at a mean rate of 0.7° C./s above        temperature T defined above (approximately 460° C.) and at a        mean rate of 0.13° C./s therebelow, in accordance with the        invention;    -   the plates of steel B, C, D are cooled at a mean rate of 6° C./s        above temperature T defined above (approximately 470° C.) and at        a mean rate of 1.4° C./s therebelow, in accordance with the        invention;    -   the plates of steel E, F, G and H which are given by way of        comparison, were cooled at a mean rate of 20° C./s above        temperature T defined above and at a mean rate of 12° C./s        therebelow.

The plates A to D have an auto-tempered martensitic/bainitic structurewhich contains approximately 10% of retained austenite, as well astitanium carbides, whereas the plates E to G have a completelymartensitic structure, the plates G and H also containing coarsetitanium carbides.

It can be seen that, although the plates A, B, C and D have levels ofhardness which are lower than those of the plates E and F, they havesignificantly higher levels of resistance to abrasion. The lowest levelsof hardness, which correspond, for the most part, to the lowest contentsof free carbon, lead to better suitability for use.

Comparison of the examples C, D, F, G and H indicates that the increasein the abrasion resistance does not result simply from the addition oftitanium, but instead from the combination of the addition of titaniumand the structure containing residual austenite. It has been found thatthe steels F, G and H whose structure does not comprise any residualaustenite have quite comparable levels of abrasion resistance, whereassteels C and D which contain residual austenite have substantiallybetter levels of abrasion resistance.

Furthermore, comparison of the pairs G and H on the one hand and C and Don the other hand indicates that the presence of residual austenitesubstantially increases the effectiveness of the titanium. In the caseof examples C and D, the increase from 0.110% to 0.350% of titaniumbecomes evident as an increase in the abrasion resistance of 56%,whereas for steels G and H, the increase is only 37%.

That observation can be attributed to the increased squeezing effect ofthe titanium carbides by the surrounding matrix when it containsresidual austenite which can be transformed into hard martensite whichexpands during operation.

Furthermore, the deformation after cooling, without planishing, for thesteel plates A or B is 6 mm/m and 17 mm/m for the steel plates E and F.These results indicate the reduction of deformation of the productsobtained by means of the invention.

The result in practice, in accordance with the extent of surfaceevenness required by the users, is:

-   -   either the products can be supplied without planishing (saving        in terms of cost and residual stresses),    -   or planishing may be carried out in order to comply with        stricter requirements in terms of surface evenness (for example,        5 mm/m), but more readily and with fewer stresses being        introduced owing to the lesser original deformation of the        products according to the invention.

1. Method for producing a workpiece, and in particular a plate, of steelwhich is resistant to abrasion and whose chemical composition comprises,by weight:0.1%≦C<0.23%0%≦Si≦2%0%≦Al≦2%0.5%≦Si+Al≦2%0%≦Mn≦2.5%0%≦Ni≦5%0%≦Cr≦5%0%≦Mo≦1%0%≦W≦2%0.05%≦Mo+W/2≦1%0%≦B≦0.02%0%≦Ti≦0.67%0%≦Zr≦1.34%0.05%<Ti+Zr/2≦0.67%b 0%≦S≦0.15%N<0.03% optionally from 0% to 1.5% of copper, optionally at least oneelement selected from Nb, Ta and V at contents such thatNb/2+Ta/4+V≦0.5%, optionally at least one element selected from Se, Te,Ca, Bi and Pb at contents which are less than or equal to 0.1%, thebalance being iron and impurities resulting from the productionoperation, the chemical composition further complying with the followingrelationships:C*=C−Ti/4−Zr/8+7×N/8≧0.095% and:Ti+Zr/2−7×N/2≧0.05% and:1.05×Mn+0.54×Ni+0.50×Cr+0.3×(Mo+W/2)^(1/2)+K>1.8 with: K=1 if B≧0.0005%and K=0 if B<0.0005%, according to which the plate is subjected to athermal quenching processing operation which is carried out in the heatfor forming in the hot state and, for example rolling heat, or afteraustenitization by means of reheating in a furnace, in order to carryout the quenching: the workpiece or plate is cooled at a mean coolingrate greater than 0.5° C./s between a temperature greater than AC₃ and atemperature of from approximatelyT=800−270×C*−90×Mn−37×Ni−70×Cr−83×(Mo+W/2) to T−50° C., the workpiece orplate is then cooled at a mean core cooling rate Vr<1150×ep^(−1.7)greater than 0.1° C./s between the temperature T and 100° C., ep beingthe thickness of the plate expressed in mm, the workpiece or plate iscooled as far as ambient temperature and optionally planishing iscarried out.
 2. Method according to claim 1, further characterized inthat:1.05×Mn+0.54×Ni+0.50×Cr+0.3×(Mo+W/2)^(1/2)+K>2.
 3. Method according toclaim 1, further characterized in that:C≦0.22% and:C*≧0.12%.
 4. Method according to claim 1, further characterized in that:Ti+Zr/2≧0.10%.
 5. Method according to claim 1, further characterized inthat:Si+Al≧0.7%.
 6. Method according to claim 1, characterized in thattempering at a temperature which is less than or equal to 350° C. isfurther carried out.
 7. Method according to claim 1, characterized inthat, in order to add titanium to the steel, the liquid steel is placedin contact with a slag containing titanium and the titanium of the slagis caused to diffuse slowly in the liquid steel.
 8. Workpiece, and inparticular a plate, of steel which is resistant to abrasion and whosechemical composition comprises, by weight:0.1%≦C<0.23%0%≦Si≦2%0%≦Al≦2%0.5%≦Si+Al≦2%0%≦Mn≦2.5%0%≦Ni≦5%0%≦Cr≦5%0%≦Mo≦1%0%≦W≦2%0.05%≦Mo+W/2≦1%0%≦B≦0.02%0%≦Ti≦0.67%0%≦Zr≦1.34%0.05%<Ti+Zr/2≦0.67%0%≦S≦0.15%N<0.03% optionally from 0% to 1.5% of copper, optionally at least oneelement selected from Nb, Ta and V at contents such thatNb/2+Ta/4+V≦0.5%, optionally at least one element selected from Se, Te,Ca, Bi and Pb at contents which are less than or equal to 0.1%, thebalance being iron and impurities resulting from the productionoperation, the chemical composition further complying with the followingrelationships:C−Ti/4−Zr/8+7×N/8≧0.095% and:Ti+Zr/2−7×N/2>0.05% and1.05×Mn+0.54×Ni+0.50×Cr+0.3×(Mo+W/2)^(1/2)+K>1.8 with: K=1 if B≧0.0005%and K=0 if B<0.0005%, the steel having a martensitic ormartensitic/bainitic structure, the structure containing carbides andfrom 5% to 20% of retained austenite.
 9. Workpiece according to claim 8,characterized in that:1.05×Mn+0.54×Ni+0.50×Cr+0.3×(Mo+W/2)^(1/2)+K>2.
 10. Workpiece accordingto claim 1, characterized in that:C≦0.22% and:C−Ti/4−Zr/8+7×N/8≧0.12%.
 11. Workpiece according to claim 1,characterized in that:Ti+Zr/2≧0.10%.
 12. Workpiece according to claim 1, characterized inthat:Si+Al≧0.7%.
 13. Workpiece according to claim 1, characterized in thatthe thickness of the plate is from 2 mm to 150 mm.